The oxygenation of free polyenoic fatty acids via the lipoxygenase pathway leads to the formation of potent mediators such as leukotrienes which are involved in a variety of physiological and pathological processes. In addition to the free fatty acid substrates, a number of 15-lipoxygenases catalyze the specific oxygenation of phospholipids. Moreover, many lipoxygenase products are rapidly incorporated into phospholipid and triglyceride of intracellular membranes. This lipoxygenase-stimulated oxygenation of membranes is a critical event in the transition of reticulocytes to erythrocytes. These findings have prompted us to study the effects of epidermal growth factor (EGF) activated lipoxygenase metabolism of fibroblast membranes. We have previously found that EGF induces the biosynthesis of lipoxygenase-derived free acid products in fibroblasts; and these compounds are an important component of the mitogenic signal. Analytical chemical characterization of the phospholipid fraction of these fibroblasts revealed that EGF treatment resulted in a significant increase in oxidized esterified products. The primary membrane product was identified as 13(S)-hydroxy-octadecadienoic acid. Further studies will address the mechanism of EGF-stimulated membrane oxygenation. We will determine whether these products arise from direct lipoxygenase metabolism of phospholipid or via re-acylation reactions. We will also examine the importance of esterified products in modulating EGF-dependent fibroblast mitogenesis.